1. Field of the Invention
The present invention relates to a cruise control method for a vehicle. More particularly, the invention relates to a cruise control method that can improve drive efficiency of a vehicle when the vehicle runs up a slope in an automatic cruise control mode.
2. Description of the Related Art
Generally, in an automatic cruise mode, vehicle speed is maintained at a constant level by an automatic cruise control system.
FIG. 1 shows such a conventional automatic cruise control system.
An automatic cruise control system comprises a vehicle speed sensor 10 for detecting a vehicle speed, a throttle position sensor 11 for detecting a throttle opening, a cruise switch 12 for setting vehicle speed at a predetermined level according to a driver's will, an electronic control unit 13 for controlling a vehicle speed to the predetermined level by receiving signals from the vehicle speed sensor 10 and the throttle position sensor 11, and a fuel injecting part 14 for injecting fuel according to a control signal transmitted from the electronic control unit 13.
When a cruise control mode is selected by operating the cruise control switch 12, the electronic control unit 13 controls the fuel injection part 14 according to vehicle speed and throttle opening signals transmitted from the vehicle speed and throttle position sensors 10 and 11. That is, by controlling an amount of fuel being injected, the vehicle speed is maintained at a constant level.
FIG. 2 shows graphs illustrating an amount of fuel to be injected with respect to vehicle speeds according to various road conditions during an automatic cruise control mode.
As shown in the drawing, when the vehicle runs on a level ground to point A, since the difference between a current vehicle speed and a target speed is very low, the amount of fuel injected is not varied substantially. That is, the fuel is supplied according to a speed difference between a current speed and a target speed.
However, as the vehicle goes up a slope, since the vehicle speed is reduced, the amount of fuel to be injected is increased. Particularly, when the vehicle reaches a mid-point of the slope, although the amount of fuel delivery is maximized, it is very difficult to increase the vehicle speed to the target speed.
In addition, when the vehicle goes down the slope, since the vehicle speed increases above the target speed, the fuel injection is stopped. In this state, when the vehicle again meets an upward slope, the vehicle speed is reduced since the coasting speed obtained when the vehicle goes down the slope cannot be maintained.
That is, when the vehicle goes up the slope, the vehicle speed is not increased to the target speed even when the fuel is injected at its maximum rate. In addition, when the vehicle goes down the slope, since the coasting speed cannot be maintained, the drive effect in cruise control mode deteriorates.